This invention relates generally to the field of manufacturing integrated circuits and specifically to process tanks employing megasonic energy.
In the manufacture of semiconductors, semiconductor devices are produced on thin disk-like objects called wafers. Generally, each wafer contains a plurality of semiconductor devices. The importance of minimizing contaminants on the surface of these wafers during production has been recognized since the beginning of the industry. Moreover, as semiconductor devices become more miniaturized and complex due to end product needs, the cleanliness requirements have become more stringent. This occurs for two reasons.
First, as devices become miniaturized, a contaminating particle on a wafer will occupy a greater percentage of the device's surface area. This increases the likelihood that the device will fail. As such, in order to maintain acceptable output levels of properly functioning devices per wafer, increased cleanliness requirements must be implemented and achieved.
Second, as devices become more complex, the raw materials, time, equipment, and processing steps necessary to make these devices also become more complex and more expensive. As a result, the cost required to make each wafer increases. As such, in order to maintain acceptable levels of profitability, it is imperative to manufacturers that the number of properly functioning devices per wafer be increased. One way to increase this output is to minimize the number of devices that fail due to contamination. Thus, increased cleanliness requirements are desired.
One method by which the industry increases the cleanliness of wafers during processing is by introducing megasonic energy to the surface of the wafers during the cleaning step. In order to utilize megasonic energy in a process tank, the tank is filled with a cleaning solution such as standard clean 1 (SC-1), standard clean 2 (SC-2), deionized water, or a diluted variant of the aforementioned chemicals. SC-1 comprises 1 NH4OH:1 H2O2:5 H2O. SC-2 comprises 6 H2O:1 H2O2:1 HCl. Once the tank is filled with the selected fluid, a source of megasonic energy (e.g., a transducer) is coupled to the fluid for producing and directing sonic energy through the fluid to the surfaces of the wafers contained therein. During megasonic cleaning, the transducer will oscillate at a megasonic rate between a negative and a positive position, generating negative and positive pressures within the fluid. As the megasonic energy oscillates, cavitation bubbles form in the fluid during negative pressure and collapse during positive pressure. This formation and collapse of bubbles removes particles from the surface of the wafers contained in the fluid.
While the use of megasonic energy does increase levels of particle removal from the wafer surfaces, prior art process tanks implementing megasonic equipment have a number of drawbacks. Due to the belief that the entire surface of the wafer must be subjected to direct sonic energy emitted from the transducers in order to properly implement megasonic cleaning, existing megasonic tanks utilize transducers whose length is equal to or greater than the diameter of the wafers to be cleaned therein. An example of this type of tank is shown in FIG. 1. As a result of the size and orientation of the transducer arrays 31, 32 in these prior art tanks, the processing chambers are much wider than the diameter of the wafers to be cleaned therein. As such, a space S between the edges of the wafers and the interior of the side walls 34 of the tank is formed. This space is undesirable because fluid in the processing chamber will have a tendency to flow through this space rather than over the surface of the wafers where it is most needed for cleaning purposes. This undesirable flow pattern results because fluid has a tendency to flow through the path of least resistance (i.e. the unobstructed space as compared to between the wafer surfaces where surface friction plays a role). This problem is especially troublesome in process tanks utilizing overflow cleaning.
As a result of the fluid not passing by the surface of the wafers, many particles that would otherwise be removed from the surface of the wafers will remain adhered thereto. Moreover, even those particles that are removed from the wafers will not be carried away from the wafers and out of the processing area. These particles can then re-contact and re-contaminate the wafer surfaces. Thus, despite the use of megasonic energy, these tanks still result in a less than optimal amount of cleaning of the wafer surfaces.
Another problem caused by these spaces arises in the chemical etching step of wafer manufacturing. The increased fluid flow rate in these unobstructed spaces causes non-uniform fluid flow over the wafer surfaces. Specifically, the flow rate of the fluid over the surface of the wafer will be greater near the side edges of the wafer than the flow rate near the center of the wafer. Non-uniform fluid flow of etching chemicals will result in non-uniform etch rates across the surface of the wafers. This in turn will result in an increased number of failed devices.
Moreover, the increased size of the processing chambers of these prior art megasonic tanks also require increased amounts of wafer processing liquid to properly clean the wafers. This increased amount of liquid results not only from the larger volume of liquid needed to fill the tank, but also from the increased liquid needed to properly remove particles form the wafer surface due to the reduced flow between the wafers. As such, the costs associated with operating these process tanks also increases.
While megasonic process tanks do exist whose transducer size and orientation do not automatically necessitate a larger tank, these tanks do not provide optimal cleaning of wafers for a variety of reasons, the foremost being that these tanks have only one transducer centered at the bottom of the processing chamber. An example of one of these tanks is shown in FIG. 2. In these megasonic process tanks, the single transducer 35 is often situated near the center of the bottom of the tank, 36 directly below the wafer stack to be cleaned or etched. When positioned this way, the transducer 35 will obstruct the fluid flow into the processing area, resulting in flow non-uniformity and decreased flow rate over the surface of the wafers.
Additionally, having a single transducer can result in other inadequacies, such as considerable heating during operation. Moreover, the transducer of the process tank shown in FIG. 2 is not in direct contact with the fluid in which the wafer is resting. As such, inadequate sonic energy will be transferred to the liquid and/or increased amounts of electrical energy will be needed to sufficiently clean the wafers.